Many natural gas reservoirs contain relatively low percentages of hydrocarbons (less than 40%, for example) and high percentages of acid gases, principally carbon dioxide, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide and various mercaptans.
Carbon dioxide acts as a diluent and, in the amount noted above, significantly lowers the heat content of the natural gas. The sulfur-bearing compounds are noxious and may be lethal. In addition, in the presence of water, these components render the gas very corrosive. The specifications for pipeline quality gas typically call for a maximum of 2%-4% carbon dioxide. Specifications for natural gas liquids recovery processes and helium recovery processes typically require less than 1% carbon dioxide. Specifications for a natural gas liquifaction plant typically require less than 100 ppm of carbon dioxide, while nitrogen rejection processes typically require less than 50 ppm of carbon dioxide. The processes requiring less than 100 ppm of carbon dioxide utilize what is commonly referred to as ultra-pure product. Because of the typical specifications, removal of acid gases from well production in remote locations is desirable to provide conditioned or sweet, dry natural gas either for delivery to a pipeline, natural gas liquids recovery, helium recovery, conversion to liquid natural gas or nitrogen rejection.
The separation of carbon dioxide from methane is difficult and consequently significant work has been applied to the development of methane/carbon dioxide separation methods. These processes can be placed into four general classes: absorption by physical solvents, absorption by chemical solvents, adsorption by solids and distillation. Distillation of mixtures of methane and acid gases including carbon dioxide can present significant difficulties. The difficulties are due to the unusual variation of the phases in equilibrium of mixtures of methane and carbon dioxide at different temperatures, pressures and ratios. The first figures of U.S. Pat. No. 4,533,372, issued Aug. 6, 1985, to Valencia et al, the disclosure of which is incorporated herein by reference as if set forth in its entirety, represents the characteristics of carbon dioxide and methane mixtures which lead to solid, liquid and vapor three phase equilibrium. Obviously, the formation of solids in a distillation column has the potential for plugging the column and its associated equipment.
Cryogenic distillations, such as disclosed by Valencia et al, provide for the separation of methane and carbon dioxide utilizing the formation of solid carbon dioxide in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature and pressure in a controlled freezing zone. The column typically consists of a lower distillation zone, a controlled freezing zone, and an upper distillation zone. It should be understood that an upper distillation zone is preferable, but not required.
The lower distillation zone is operated normally at a temperature and pressure at which substantially no carbon dioxide solids are formed to produce an enriched carbon dioxide liquid bottoms stream and vapor feedstream of methane and carbon dioxide, which progresses to the controlled freezing zone. The vapor feedstream is contacted with at least one liquid feedstream containing methane in a controlled freezing zone at a temperature and pressure producing both carbon dioxide-containing solids and a methane-enriched vapor stream. The methane-enriched vapor stream formed in the controlled freezing zone is contacted with a methane-enriched liquid stream in an upper distillation zone at a temperature and pressure at which substantially no carbon dioxide solids are produced. The methane produced in the upper distillation zone meets most specifications for pipeline delivery. However, more stringent specifications for higher purity natural gas exist for applications such as helium recovery, cryogenic natural gas liquids recovery, conversion to liquid natural gas, and nitrogen rejection. The more stringent specifications may be met by increasing the height of the upper distillation zone and/or increasing the liquid methene reflux.
During start-up operations of cryogenic distillation utilizing a controlled freezing zone, a liquid reflux must be generated and fed back to the distillation column to provide cooling for the system and begin the reflux necessary for efficient separation at operating temperatures. The liquid reflux may be fed at the top of the tower, to the controlled freezing zone, or to both the top of the tower and the controlled freezing zone during start-up.
During start-up operations, it is necessary to prevent carbon dioxide solidification outside the controlled freezing zone while the tower is cooled to operating temperatures. Currently, cryogenic distillation processes are started by using an essentially pure methane feedstream or by injecting small quantities of propane, heavier hydrocarbon or methanol into the system. The liquid reflux generated during start-up using the pure methane feedstream does not contain any carbon dioxide, while the presence of hydrocarbons including propane or methanol suppresses the solidification of any carbon dioxide present in the generated liquid reflux.
Cryogenic distillation utilizing a controlled freezing zone may be desirable at locations where pure methane, propane or methanol are unavailable. One such location would be a remote field where a cryogenic distillation could be used to generate fuel gas. Therefore, the need exists to provide a method and apparatus to start-up a cryogenic distillation process utilizing a controlled freezing zone without the need for pure methane or other additives used in start-up to suppress carbon dioxide solidification. The need for an ultra pure product produced with relaxed control standards also exists.
This invention relates generally to the start-up process of a cryogenic distillation column with a controlled freezing zone using methane feed containing acid gases and after start-up the further purification of the product stream. Specifically, the method of the invention supplements cryogenic distillation. The first step is stripping or removing the acid gas components from the upper distillation zone overhead stream. The second step is refluxing the stream stripped of acid gases to the cryogenic distillation column during start-up of the column. Another related process of the invention is stripping the acid gas components from the vapor phase of the upper distillation zone overhead stream to produce ultra pure product during operation after start-up.
Another process of the invention is transforming the cryogenic distillation column from start-up conditions to operating conditions. The method of the related process has multiple steps. The first step is stripping or removing the acid gas components from the upper distillation zone overhead stream. The second step is refluxing the stream stripped of acid gases to the cryogenic distillation column during start-up of the column. These steps are continuously repeated until the cryogenic distillation column approaches operating conditions. The next step is to redirect the upper distillation zone overhead stream from the stripping apparatus to a separator. In the separator the upper distillation zone overhead stream is divided into vapor and liquid components. The next step is stripping or removing the acid gas components from the vapor component of the overhead stream. The final step is refluxing or returning the liquid component of the overhead stream to the distillation column.